Ultrasonic transducer pairs, comprising a transmitter and a receiver, are commonly applied to detect air in a fluid delivery line segment as part of medication infusion systems, such as PLUM A+™, GEMSTAR™ and SYMBIQ™. The sensors are physically located on opposite sides of a fluid delivery line segment and the presence of air in the fluid delivery line causes an acoustical open circuit which substantially attenuates the detected signal. When fluid is present, propagation of the acoustic signal is efficient and produces a large electrical signal via the receiver circuit.
Detection of air in the fluid delivery line segment is typically performed on the basis of a fixed air-fluid boundary or threshold that is applied to the sensor voltage or current signal. Any signal on the fluid side of the threshold is classified as representing the presence of fluid in the infusion line and any signal on the air side of the threshold is classified as representing air. Typically a fixed threshold is applied that is specific to the infusion pump set and sensor arrangement.
When air is infused past the sensor pair, the presence of a stationary fluid droplet that bridges the gap between the transducers may lead to an acoustic short circuit. This can produce an absolute sensor signal similar to that of a fluid and result in a false negative indicating the presence of fluid when air is actually disposed over the sensor. This is problematic because the air sensor signal, indicating that air is in the infusion line, is typically used to produce an air-in-line alarm, to pause the pumping mechanism, and to reduce the potential for the infusion of air into a patient's vascular system.
Additionally, air detection systems in infusion delivery lines typically assume a fixed, constant dynamic range for the sensor voltage signal. However, if the dynamic range of the sensor voltage signal increases, due to typical system variation, the system may become less sensitive to the presence of air within the system. This may lead to the system mistakenly determining that fluid is in the system when air is actually in the system.
An infusion system and method is needed which will accurately determine the presence of a stuck fluid droplet between a sensor pair to indicate the presence of air in the system. Additionally, an infusion system and method is needed which will dynamically update, in real-time, the dynamic range for the sensor voltage signal to more accurately determine the presence of air within the system.